Braking systems with dual amplification ratios generally include a master-cylinder intended, when a brake pedal is operated, to subject the brake fluid to an increased pressure transmitted by means of suitable pipes to the brakes of the vehicle so as to operate them. Between the master-cylinder and the brake pedal is interposed a braking force amplifier, able to multiply the force applied by a driver to the brake pedal by a first amplification ratio while this force does not exceed a certain threshold, and then by a second amplification ratio beyond this threshold.
In known manner, a braking force amplifier includes a casing, a diaphragm separating the casing into a front chamber which is provided with means for connection to a vacuum source and a back chamber.
The braking force amplifier also conventionally includes a mobile assembly formed on the one hand by a thrust assembly intended to transmit the amplified force at the output and on the other by a piston intended to communicate to the thrust assembly the additional force generated by a pressure difference between the front chamber and the back chamber. The piston includes a plunger mounted to be axially displaceable within it and able to come to bear against a reaction disc. This plunger transmits the force applied to the brake pedal to the thrust assembly by means of the reaction disc.
The thrust assembly is composed principally of a sleeve, and a push-rod bearing against the piston by means of the reaction disc and mounted to be axially displaceable within the sleeve. The thrust assembly also includes a spring intended to oppose the penetration of the push-rod into the sleeve.
The reaction disc is interposed between on one side a bearing surface of the push-rod and on the other side bearing surfaces of the plunger and of the piston respectively.
Under the influence of an operating rod connected to the brake pedal and receiving the braking force or input force, the plunger and the piston jointly apply a thrust force to the push-rod by means of the reaction disc. The push-rod, under the joint influences of the thrust force and the return force of the spring, is displaced axially in the sleeve of the thrust assembly between a first braking state corresponding to a first ratio of amplification of the input force and a second braking state corresponding to a second ratio of amplification of the input force.
In these known braking force amplifiers, the adjustment of the threshold of the input force F0, for which the passage between the first and the second braking states takes place, is effected by selecting a spring the constant of elasticity of which is more appropriate, which is irksome.